The present invention relates to an electronic power supply for a synchronous motor with permanent-magnet rotor with two pairs of poles, supplied directly from the mains.
A synchronous motor with permanent-magnet rotor has many applications, especially in the civil sector, where the power levels involved are low or, as an approximate indication, lower than 100 W.
In their most basic structure, these motors have a fixed stator part and a part which rotates about its own axis of symmetry and is termed rotor. In these motors, the stator is constituted by an iron core which is shaped like a tuning fork and on which spools are fitted which support the windings, powered by the mains with an electronic device interposed.
The rotor is constituted, in its simplest form, by a cylinder of magnetic material which is rigidly coupled to the rotation shaft. The stator windings, supplied by the mains, produce a magnetic field which interacts with the magnetic poles of the rotor, causing the rotation of the rotor and therefore of the device connected thereto, which can be for example the impeller of a pump.
The advantages of the synchronous motor with permanent-magnet rotor with respect to an induction motor are both technical and economical. Technically, this type of motor is more compact, for an equal power level, and is always far more efficient than an asynchronous motor.
For instance, the U.S. Pat. No. 5,276,392 to Beckerman or the U.S. Pat. No. 5,325,034 to Reynolds both relate to a single phase AC motor including at least a winding connected in series with a phase shift capacitor and a second winding connected to a switch.
This kind of induction motor is asynchronous and there is no need to know the rotor position. Moreover, the rotor doesn't have permanent magnets and doesn't require a specific electronic control for driving the starting phase. However, the motor structure is complex, expensive and not suitable for low power applications.
Another reason to prefer synchronous motors is their low cost and the simple structure of the rotor and of the stator. This type of motor is single-phase, since the only winding is supplied with power by the mains voltage.
The U.S. Pat. No. 5,434,491 to this applicant relates to a synchronous motor with a permanent magnet rotor and including an electronic device for driving the starting phase of the motor.
However, this motor has application limits, particularly when low noise in operation is required. Because of its operating characteristic, the generated torque is in fact not constant at each instant during rotation.
In particular, the torque oscillates about a medium value and the oscillation frequency depends on the frequency of the supply voltage.
The torque that oscillates about the medium value can be considered as the sum of a constant term, which is responsible for moving the load, and of a pulsed term, which produces vibrations in the motor.
Additionally, the asymmetry of the stator pack means that there is a preferential direction for the attraction force which is in any case applied between the stator, constituted by iron laminations, and the rotor, which is made of magnetic material. This axial interaction and the pulsed nature of the torque, lead to pulsed stresses and therefore to vibrations which are generated in the stator of the motor.
The stator is always rigidly coupled to a supporting structure and therefore the structure is affected by these vibrations unless damping is provided, assuming of course this is possible; such damping is in any case expensive. In some applications, for example in a circulation pump for heating systems, these vibrations are in the audible frequency range and therefore produce an undesirable and unacceptable noise. These vibrations can be reduced, at least theoretically, by means of different electronic or mechanical refinements, but such refinements are expensive and scarcely reliable and in any case are only palliatives, since they tend to reduce the effect, but do not contrast the cause, of the noise.